Referring to a sectional view of FIG. 11, an aluminum electrolytic capacitor includes, as basic components, a capacitor element 1 including a pair of lead terminals 2, 2. As shown in FIG. 12, the capacitor element 1 is formed by winding, via not-shown separator paper, in a spiral shape, an anode foil 1a and a cathode foil 1b made of aluminum material and having the lead terminals 2 attached thereto.
The capacitor element 1 is housed in an armor case 3 having a bottomed cylindrical shape together with a predetermined electrolytic substance. An opening section of the armor case 3 is sealed by a rubber seal 4. Terminal passage holes 5, 5 are drilled in the rubber seal 4. Respective distal end portions of the lead terminals 2, 2 are drawn out to the outside through the terminal passage holes 5, 5.
Actually, the rubber seal 4 is housed in the armor case 3 together with the capacitor element 1 in a state in which the tip of the rubber seal 4 is attached to the lead terminals 2, 2 earlier. The rubber seal 4 is hermetically fixed in the opening section of the armor case 3 by caulking of a lateral reduction groove 3a of the armor case 3, which is formed later, and an end edge 3b of the armor case 3.
Usually, an aluminum case is used as the armor case 3. Isobutylene-isoprene rubber or the like is used as the rubber seal 4. Usually, non-aqueous or aqueous electrolyzed liquid is used as the electrolytic substance. However, a solid electrolyte is sometimes used.
As shown in FIG. 12, the lead terminal 2 includes a tab terminal 21 and an outside lead wire 22. The tab terminal 21 is made of an aluminum material and includes a flat section 21a press-molded in a battledore shape and a round bar section 21b. 
The tab terminal 21 of this type can be obtained by cutting an aluminum round bar material at predetermined length and pressing one end side of the round bar material. The flat section 21a is attached to the anode foil 1a and the cathode foil 1b by caulking needles, welding, or the like.
Usually, a copper-coated steel wire (CP wire) is used as the outside lead wire 22. To improve solderability to a circuit board, the outside lead wire 22 includes a plated layer on the surface. In the case of Pb (lead) free, Sn 100% plating, Sn/Bi (0.5%) plating, or the like is mainly applied to the plated layer.
The outside lead wire 22 is smaller in diameter than the round bar section 21b of the tab terminal 21 and is welded to an end face of the round bar section 21b. A welded section of the outside lead wire 22 is denoted by reference numeral 23.
Incidentally, when the plated layer on the surface is the Sn 100% plating, the plated layer other than the welded section 23 is stable. However, in the welded section 23, Al, An, Cu, Fe, and the like are mixed. When the welded section 23 is exposed to the outside air, stress acts on an Sn layer because of hydration and oxidation reaction of Al. An Sn whisker (a whisker-like crystal substance) 23a intensely occurs and grows.
In the Sn/Bi (0.5%) plating, although the growth of the whisker is relaxed than that in the Sn 100%, the whisker also occurs. When the growth of the whisker is conspicuous, there is a risk that the whisker scatters on the circuit board and, in the worst case, shirt-circuits an electronic circuit.
Therefore, in order to suppress the occurrence of the whisker as much as possible and prevent the scattering of the whisker to the outside, in Patent Documents 1 and 2, as shown in FIG. 13, the applicant proposes to coaxially connect, as the terminal passage hole 5 of the rubber seal 4, a large-diameter round bar mating hole 51, in which the round bar section 21b of the tab terminal 21 is fit, and a small-diameter lead wire passage hole 52, through which the outside lead wire 22 is inserted, and set a hole diameter φ2 of the lead wire passage hole 52 smaller than an outer diameter φ1 of the outside lead wire 22 (φ2<φ1) to shut off the welded section 23 from the outside air.
According to Patent Documents 1 and 2, the scattering to the outside of the whisker 23a that occurs in the welded section 23 is prevented. However, on the other hand, since the hole diameter φ2 of the lead wire passage hole 52 is smaller than the outer diameter φ1 of the outside lead wire 22 (φ2<φ1), when the outside lead wire 22 is forcibly inserted through the lead wire passage hole 52, excessively large stress is applied to the capacitor element 1. Because of the excessive stress, the characteristics of the capacitor element 1 are sometimes deteriorated.
In order to solve this point, Patent Document 3 proposes to form, in the lead wire passage hole of the rubber seal, a funnel-shaped cylindrical body made of a thin film gradually reduced in diameter toward the outer side of the capacitor.
Patent Document 4 proposes to integrally form, in the lead wire passage hole of the rubber seal, a sealing plug body having a slit for inserting the lead terminal and draw out the lead terminal to the outside via the slit.
As a technique similar to Patent Document 4, Patent Document 5 proposes to integrally form, in the lead wire passage hole of the rubber seal, a sealing plug made of a thin wall for closing the passage hole and draw out the lead terminal to the outside piecing through the sealing plug.